1. Field of the Invention
The present invention relates to imaging devices and, more particularly, to methods and systems for compensating for defective pixels and pixel values.
2. Description of the Related Art
Conventional integrated circuit imaging devices include an array of light detecting elements or pixels which are interconnected to generate an analog signal representation of an image illuminating the device. Two common examples of conventional integrated circuit imaging devices are a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS) image sensing device. Conventional imaging devices typically include one or more light detecting elements and charge storage elements. Such integrated circuits employ a light detecting element, such as a photodiode or phototransistor, that is capable of conducting current approximately proportional to the intensity of the light illuminating the element and a charge storage element. The collection of the signals from the pixels represents the image of the scene viewed by the array.
In order to produce a color image, the imaging devices separate the light into various color components by filtering the light before the light strikes the light detecting elements. The array of light detecting elements is often deposited with a filter layer such that neighboring pixels may have different color filters and organized in a particular pattern.
Because each pixel is typically only capable of detecting a single color, conventional imaging devices utilize a process by which all of the color components are reconstructed for each pixel in order to maintain the original unfiltered array resolution. To reconstruct the color components, conventional imaging devices use a process of color interpolation that is performed after an analog signal associated with each pixel has been digitized. The conventional process of color interpolation performed after an analog signal associated with each pixel has been digitized requires conversion from analog to digital (A/D) and may require extensive computations in order to achieve a high quality color presentation of the image. The A/D conversion and extensive computations may require hardware, such as analog-to-digital (A/D) converters, memory, processors and software. The hardware and software may add to the complexity, size and expense of the imaging device and reduce the speed of the imaging process.
Each pixel represents a sample of the scene and hence is a data value in the two-dimension image produced by the imaging system. Defective pixels, commonly referred to as ‘bad pixels,’ are caused by an array defect and do not provide the correct light intensity value. Bad pixels appear as image artifacts that can reduce the image quality significantly. In particular, a bad pixel produces an output signal that significantly deviates from the mean output level of adjacent pixels when the exposure level of all pixels is unified. Pixels that are significantly brighter than adjacent pixels in a unified dark frame are commonly referred to as a ‘hot pixels,’ while pixels that are significantly darker than adjacent pixel in a unified bright frame are commonly referred to as ‘dead pixels.’
The defective pixels are typically distributed in a random manner. However, a ‘bad column’ (i.e., a complete column is defective) or a blemish (i.e., a cluster of neighboring pixels is defective) may occur and are typically discarded by the manufacturer. Sensor arrays that contain random defective pixels in an amount that does not exceed a given limit are released, and the remaining bad pixels are sometimes be corrected in some other manner.
Some CCD and CMOS integrated circuit color imaging devices employ a process of bad pixel detection and correction. Conventionally, the bad pixel detection step is performed off-line by the manufacturer, before the imaging device is shipped. A bad pixel list is typically stored in an EEPROM lookup table. When the imaging device is later used in a product, such a camera, the lookup table is read to locate the bad pixels. Once the bad pixel locations are read, the values of the bad pixels are discarded when constructing an image based upon the imaging device readout. The bad pixel correction is performed by substituting the bad pixel value with the value of a pre-specified other pixel, wherein a lookup table created during the manufacturing process is used to identify the pre-specified other pixel whose value is to be used. This conventional correction step is typically performed after the analog signal for each pixel has been digitized.
The conventional bad pixel detection and correction process described has several significant drawbacks. For example, some of the array defects that cause bad pixels are temperature and gain dependent, and thus may appear and disappear during operation. Because these bad pixels are not consistently “bad,” they may not be detected by the manufacturer, and hence, may not be corrected. In addition, because the detection step is performed off-line by the manufacturer, a non-volatile storage device is needed to store the defect pixel list. Adding a storage unit disadvantageously increases the cost and complexity of the system.